The present invention relates to a probe for testing of integrated circuits or other microelectronic devices.
One type of probe utilizes a spaced-apart array of slender needles to contact pads on a device under test (DUT). A signal is provided to the DUT, and the voltages and/or currents at the selected nodes are routed to measurement equipment. A problem encountered with such measurement systems, particularly at high frequencies, is that the close proximity between the needle tips creates inductance that can interfere with accurate measurements. Though this inductance can be reduced by limiting the isolated portion of the probe tips to the region immediately surrounding the DUT, practical considerations make such a design difficult.
Probe structures have been developed to compensate for the inductance at the probe tips. One such design is exemplified by Lockwood et al., U.S. Pat. No. 4,697,143. Lockwood et al. disclose a ground-signal-ground arrangement of strip like conductive traces formed on the underside of an alumina substrate so as to create coplanar transmission lines. These coplanar transmission lines extend from the pads of the DUT at one end to a coaxial cable at the other end. The associated pair of ground traces on each coplanar transmission line is connected to the outer conductor of the coaxial cable and the interposed signal trace is connected to the inner conductor. Areas of wear-resistant conductive material are provided to reliably establish an electrical connection with the respective pads of the DUT. Layers of ferrite-containing microwave absorbing material are mounted about the substrate to absorb spurious microwave energy over a major portion of the length of each ground-signal-ground trace pattern. In accordance with this type of construction, a high frequency impedance (e.g., 50 ohms) can be presented at the probe tips to the device under test. Thus broadband signals of eighteen gigahertz or less can travel with little loss across the coplanar transmission lines formed by each ground-signal-ground trace pattern.
The probing system of Lockwood et al., however, is insufficient to effectively probe non-planar surfaces. Such surfaces might result, for example, if the pads of the DUT differ in height, if a loose metallic particle of minute dimension adheres electrostatically to the surface of one of the pads of the DUT so as to form a non-planar surface irregularity, or when the plane of the DUT is inadvertently tilted slightly with respect to the plane of the coplanar tips of the probing assembly. Further, proper alignment between the needles and the DUT requires careful placement of each needle, a time consuming process.
The alignment limitation between the needles was addressed by Godshalk, U.S. Pat. No. 5,506,515. Godshalk discloses a ground-signal-ground finger arrangement attached to a coaxial cable, as in Lockwood. The fingers, however, are originally formed in one piece, joined together by a carrier tab at the contact ends. Once the fingers are attached to the coaxial cable, the carrier tab is severed and the contact fingers appropriately shaped for contact with the DUT. Godshalk discloses that the relative position of each finger is held in alignment first by the carrying tab, and then by the coaxial cable. Unfortunately, Godshalk's design is limited in that the close placement of a coaxial cable to the finely spaced geometry of the DUT places a limit on the number of coaxial cables, and hence contact fingers, that may be used effectively in the probe. Further, a probe having multiple adjacent coaxial cables, each of which has different flexibilities, may lead to insufficient contact with some of the nodes on the DUT.
Another class of probes that provide clean power to circuits at low impedance are generally referred to as power bypass probes. Another configuration that has been developed to counteract the inductance at the tips of a probe assembly is a power bypass quadrant. The power bypass quadrant minimizes such inductance by providing integrated capacitors or resistor-capacitor networks within the probe.
Strid, U.S. Pat. No. 4,764,723, discloses a power bypass quadrant probe that utilizes an array of ceramic fingers coated with a thin gold or polyimide film to make contact with the DUT. The test signals are routed through a power bypass structure consisting of an RC network. Because of the small geometries near the DUT, the capacitors are located far away from the probe tip, which potentially decreases performance. In addition, the ceramic contact fingers tend to break during probing, particularly when the probe overshoots the contact pads. Further, probing pads that are not coplanar is exceedingly difficult because the ceramic contacting fingers lack flexibility.
Boll et al., U.S. Pat. No. 5,373,231 disclose a probe that includes an array of blades to contact the pads of a DUT. The array of blades extend from a transmission line network traced on a circuit board. An RC network is provided on the circuit board to provide the requisite power bypass, and in some instances, flexible capacitors are located close to, or between the contact blades. Because of the limited geometries near the DUT, the capacitance of the capacitors interconnected between the blades are small, and alone are insufficient to adequately eliminate circuit inductance. Accordingly, a second bank of capacitors with larger values are located away from the probe tip where space is available. Probes utilizing flexible capacitors between the closely spaced blades of the probe have proven to be of limited mechanical durability.
What is desired, therefore, is a configurable, multi-contact probe for high frequency testing of integrated circuits or other microelectronic devices that reduces the inductance at the probe tip to levels acceptable for measurement over a wide range of frequencies. The probe should be sufficiently durable and flexible to reliably and repeatedly probe substantially non-planar devices over time. It is further desired that the probe be easily aligned with the contact points on the device to be tested and that the probe be capable of simultaneously probing a number of such contact points.